This invention relates to an improved process for preparing, in a single hydrogenation step, saturated omega-amino acids from olefinically unsaturated omega-aldehyde acids.
This invention also refers to the omega-amino acids thus obtained.
The industrial importance of these products is well known and does not require any evidence.
It will be enough to recall for example the wide use of these amino acids as monomers for the manufacturing of polyamides used as plastics in the textile field for the manufacture of polyamide fibers and yarns.
The omega-amino acids to which this invention refers are defined by the general formula (I) H.sub.2 N--(CH.sub.2 --).sub.n+1 COOH wherein "n" is an integer varying from 4 to 16 and preferably 4-10.
From U.S. Pat. No. 4,054,588 it is known the conversion, in a single hydrogenation step of an unsaturated, straight chain .omega.-aldehyde acid into the corresponding .omega.-amino acid. More particularly in the U.S. Pat. No. 4,054,588 the unsaturated omega aldehyde acid is converted to the corresponding imino acid and then is reduced with H.sub.2 both at the imino function and at the ethylenic double bond in a single step in the presence of a hydrogenation catalyst comprising a transition metal of one of the first two subgroups of group VIII of the periodic system of the elements, that is one chosen from Ru, Os, Co, Rh, Ir. The best results are obtained by using ruthenium and rhodium supported on carbon, obtaining yields of about 80% or somewhat higher of the stoichiometric yield. It was also known from the Italian Pat. No. 998227 that carrying out the reduction of the imine function of the unsaturated aldehyde acid in water and alkali and using a nickel catalyst, under particular conditions the unsaturated amino acid is thus obtained, while changing the conditions, for example increasing temperature, resulted in addition to small amounts of the saturated amino acid, in the production of many by-products as well.
It has now been surprisingly found that this conversion, in a single hydrogenation step, of the unsaturated straight chain acid to the corresponding saturated amino acid, can be carried out with high yields (.gtoreq.90%) and with much less production of undesired by-products in particular operating conditions also in the presence of metallic nickel or a nickel compound, thereby obtaining not only improved yields with respect to Ru, Rh, etc. but also advantages of economical nature. A further advantage will result clearly considering that by carrying out the reaction in two hydrogenation steps as disclosed in the Italian Pat. No. 998227 it is necessary to isolate, regenerate and recycle two catalysts, with possible mixing thereof, in which case, not only may the product be lost, but the whole catalyst may have to be necessarily replaced as well.